One of the problems is that custom maps and mods do not auto-download for online multiplayer games, decreasing people's desire to make them.  Also, it seems like everything needs to get updated everytime Sins has a major update.  See if you can find a mod called "Seven Deadly Sins".  I've never played it since I only play online multiplayer and I don't want to mess up my game install, but if you only intend to play it in single player, that's supposed to be a great mod.